The present invention relates to cutaneous gas sensors and is directed more particularly to an improved temperature control system which protects a patient from exposure to cutaneous gas sensors which are operating at excessive temperatures, and which significantly reduces the time that elapses between the application of the sensor to the patient and the generation of useful data.
Among the non-invasive patient monitoring instruments which have been developed recently is an instrument known as a cutaneous gas sensor. Gas sensors of this type make use of known gas detection techniques to measure the partial pressure of a gas, such as oxygen or carbon dioxide, which diffuses outwardly through the pores of a patient's skin. Cutaneous gas sensors have also been developed which simultaneously measure the partial pressure of both oxygen and carbon dioxide. One cutaneous gas sensor of the latter type is described in "Cutaneous Blood Flow and its Relationship to Transcutaneous O.sub.2 /CO.sub.2 Measurements", by A. V. Beran, et al., "Critical Care Medicine", Vol. 9, No. 10, pp. 736-741 (1981).
Because the rate at which blood gases diffuse through human skin is related to the temperature of the skin, cutaneous gas sensors include heating elements whereby the temperature of the skin at the measurement site may be maintained at a temperature that is higher than normal body temperature. A typical gas sensor will, for example, be maintained at a temperature, such as 42.degree. to 43.degree. C., which is several degrees higher than the normal human body temperature of 37.degree. C. This elevated temperature is maintained by a closed loop temperature control circuit which continuously compares the actual sensor temperature with a desired setpoint temperature and increases or decreases the current flow through the heating element as necessary to maintain the desired temperature.
Existing temperature control circuits for cutaneous gas sensors have two important deficiencies which limit their usefulness. One of these is that temperature control circuits lack adequate provision for shutting off the flow of current through the heating element in the event that one or more parts of the circuit malfunction. Such a shutoff is extremely important because cutaneous gas sensors are often applied to patients, such as infants or comatose individuals, who are unable to remove a gas sensor the control circuitry of which has failed in a way that causes it to apply excessive current to the heating element. This excessive current can result in serious injury, particularly in cases in which gas sensors are allowed to operate unattended for hours at a time.
If, as is often the case, the gas sensor operates under the control of a programmed microcomputer, the solution to the problem of automatically shutting off the flow of heater current is made more difficult by the fact that microcomputers can malfunction as a result of power line transients, electrical noise and cosmic rays. In some cases, these malfunctions can cause the microcomputer to become unable to limit the flow of heater current, or even to initiate the flow of excessive heater current.
Another deficiency of presently available cutaneous gas sensors is that they take a long time to come into thermal equilibrium with a patient after first being applied thereto. When, for example, a sensor at a temperature of 42.degree. C. is first applied to a patient whose body is at a temperature of 37.degree. C., the temperature of the part of the patient to which the sensor is applied (the measurement site) will initially remain below the temperature at which useful data can be taken. This condition will continue until enough additional power can be applied to the heater to raise the temperature of the measurement site to 42.degree. C. Since the amount of power supplied to the sensor depends upon the difference between the actual and desired temperature of the measurement site, the power supplied to the heater is gradually reduced as the temperature of the measurement site approaches 42.degree. C. As a result, the temperature of the measurement site tends to approach the desired value in an asymptotic manner. During this asymptotic approach, 30 minutes or more may pass between the time that the sensor is first applied and the time that useful data can be produced.